Breast cancer is the most frequently diagnosed malignancy among women other than skin cancer, and the second leading cause of mortality in women (following lung cancer). The American Cancer Society estimates that over 200,000 new invasive cases of breast cancer will occur each year, and more than 40,000 deaths from breast cancer among women in the United States. Early detection is the most important factor to survival, with a survival rate of 96%, if the cancer is found early. As a result, breast MR imaging has become of more interest in the past years, and it is now a tool that is widely used in clinical routine as an important method to assess especially the difficult cases, where conventional mammography and ultrasound are at their limits.
Mammography has been found to often miss breast cancer in its early stages when it is most amenable to treatment and most likely to be cured. Between 10-30% of women who have breast cancer and undergo mammography have negative mammograms, and in about two-thirds of these cases, the radiologist failed to detect retrospectively evident cancer. Such misses have been attributed to the subtle nature of the visual findings, poor image quality, or oversight by the radiologist. Mammography is also disadvantageous in that it is limited in individuals who have breast implants, and is not as accurate in younger individuals whose breast tissue tends to be denser. In addition, mammography exposes individuals to ionizing radiation that may increase their risk of developing breast cancer. Mammography also requires significant compression of the breast tissue that many individuals find painful, leading them to avoid mammography.
Magnetic Resonance Imaging (MRI) has become an important non-invasive medical technique over the past decade. For example, U.S. Pat. No. 6,411,837 discloses a method for high-resolution magnetic resonance tomography of the female breast, U.S. Pat. No. 6,468,231 discloses a method and device for detecting changes in mechanical and structural properties of breast tissue, and U.S. Pat. No. 6,363,275 discloses a device for detecting and for treating tumors using differential diagnosis. Each is herein incorporated by reference.
MR imaging uses a strong direct current magnetic field in conjunction with tunable gradient magnetic fields to spatially control locations at which the net sum magnetic field reaches a pre-selected value. As the magnetic bias fields are varied spatially, a series of radio frequency (RF) pulses are applied. When the RF energy is at a resonance frequency of sample atoms of a particular species and surroundings, those sample atomic nuclei absorb the RF energy and are excited to a higher spin state. The excited spin state then decays to a lower energy state of excitation and the decay is accompanied by an emission of an RF pulse. The RF of a nucleus (nuclear magnetic resonance, or “NMR”), and its resulting signal depend on a number of factors, including mass, density, dipole moment, relaxation frequency, as well as chemical bonding and electrostatic potential of its surroundings. To enhance the contrast between tissues within an organism, one or more contrast agents may be introduced into an individual's body prior to MRI analysis.
The NMR signals are detected using one or more radio-frequency (RF) “coils.” The term “coil” is also commonly used to refer to the electrical part of the device and its housing or support structure. The size of the local coil is kept small to allow them to be easily fit to the patient on the MRI device, and to enable imaging of only the imaging volume of interest, since imaging regions that are not required adds noise to the acquired signal unnecessarily. However, the smaller the size of the local coil, the smaller its field of view, or sensitivity profile. Imaging of larger areas using the smaller coils requires the use of multiple small coils, either simultaneously in a combined manner or by moving the coil between imaging acquisitions. Such coils can be operated individually or as multi-coil arrays. Combining signals from multiple coils can yield improvements in SNR. However, one of the challenges associated with using multiple coils for imaging is the fact that the fields of individual coils may interact, resulting in coil-to-coil coupling. Such interactions tend to reduce the coil quality factor, or Q.
Dynamic contrast enhanced MR (DCE-MRI) breast imaging has shown promising results in its ability to detect breast abnormalities (Heywang et al., J. Comput. Assist. Tomogr. 10:199-204 (1986); Kaiser et al., Radiology 170:681-686 (1989); Heywang et al., Radiology 171:95-103 (1989); Orel et al., Radiology 190:485-493 (1994)). Architectural features visible after enhancement have a high correlation with cancer (Nunes et al., Radiology 202:833-841 (1997); Nunes et al., Radiology 219(2):484-494 (2001)) and the enhancement dynamics have also been shown to be highly correlative with benign or malignant lesions (Kuhl et al., Radiology 211:101-110 (1999)). However, there previously had to be a decision to either acquire MRI data with high temporal and low spatial resolution, or data with high spatial and low temporal resolution. Combined interpretations have been shown to improve diagnostic performance over each separate approach (Schnall et al., Academic Radiology 8(7):591-597 (2001); Vomweg et al., Medical Physics 30(9):2350-2359 (2003); Szabo et al., European Radiology 14(7):1217-1225 (2004)). However, even for a unilateral breast study, the simultaneous acquisition of both high spatial resolution data for architectural analysis and high temporal resolution data for contrast kinetic classification is difficult due to their diverging demands (Dougherty et al., ISMRM 13th Scientific Meeting and Exhibition, page 86, Miami Beach, May 2005).
The high-resolution imaging needed to distinguish features necessary for architectural interpretation requires a relatively long time to acquire. For example, to image the entire breast, a 3-dimensional acquisition of 32 slices with a sampling matrix of 512×384, takes ˜2 minutes in a typical clinical exam. Acquiring fewer slices or reducing the matrix size will speed acquisition but at the cost of coverage or spatial resolution. The importance of using a high frame rate for enhancement dynamics analyses was shown by (Lucht et al., J. Magn. Reson. Imaging 19(1):51-57 (2001)), who have reported a significant increase in diagnostic performance when using 28 points as compared to three time points.
Keyhole rectilinear k-space acquisitions have also been proposed (van Vaals et al. J. Magn. Reson. Imag. 3:671-675 (1993); Jones et al., Magn Reson Med 29:830-834 (1993)). In the keyhole technique, only the low spatial frequencies along the phase encoding direction are acquired at short intervals, and the full resolution images are reconstructed by using the high spatial frequencies from a reference dataset. However, this acquisition scheme causes the mixing of the constantly updated low spatial frequency data with the high frequency data acquired at different time periods, potentially resulting in blurring of the enhancing structures in the phase encoding direction. Other related acquisition schemes have been developed to help reduce these artifacts (Parrish et al., Magn. Reson. Med. 1995; 33:326-336 (1995); Korosec et al., Magn. Reson. Med 36:345-351 (1996); Mistretta et al., Magn. Reson. Med. 40:571-581 (1998)). However, the inevitable mixing of old and new data that are non-contiguous in time still occur, and may cause measurement errors.
A further consideration with coil systems is their ability to operate in a parallel MR imaging mode. Parallel imaging methods, such as SMASH (Sodickson et al., Magn. Reson. Med. 38(4):591-603 (1997)) or SENSE (Pruessmann et al., Magn. Reson. Med. 42(5):952-962 (1999)) have gained attention in the last few years as methods to reduce scan time, and thus, improve temporal resolution without sacrificing spatial resolution. In these approaches, spatial information carried by the placement of multiple receiver coils can be used to reduce the number of phase encoding steps required for traditional spatial encoding. Based on the sensitivity profiles of these coils operating independently, a reconstruction algorithm can be implemented that enables reconstruction of a full image volume in a fraction of the conventional image acquisition time.
Researchers have shown reduction factors of 2-3 using SENSE encoding in application to breast imaging (van den Brink et al., European J. Radiology. 46(1):3-27 (2003); Friedman et al., AJR 184:448-451 (2005)). Larkman et al. (J. Magn. Reson. Imaging 13:313-317 (2001A)) have previously described the use of multi-coil arrays for separation of signals from multiple, simultaneously excited slices, but was not adapted to multiple 3D volumes. With such parallel imaging methods, temporal resolution can be increased. However, with greater acceleration factors the SNR is concomitantly decreased and the time resolution is still insufficient to adequately sample the contrast kinetics. Dougherty et al., “Parametric Mapping of Contrast Kinetics from Rapid Radial MR-DCE Breast Images,” Abstract, ISMRM 14th Scientific Meeting and Exhibition, Seattle, May 2006.
Under-sampled radial imaging has also been investigated as a way to reduce imaging time (Joseph et al., Med. Phys. 10(4):444-449 (1983); Peters et al., Magn. Reson. Med. 43:91-101(2000); Vigen et al., J. Magn. Reson. Med. 43:170-176(2000)). It has been shown that the number of projections can be greatly reduced using this method, while preserving spatial resolution and reducing the scan time. Further, by interleaving the radial acquisitions, a method that allows image reconstruction at two different resolutions has also been described (Proksa et al., “Multi-resolution MRI.” In: Proc 5th Scientific Meeting ISMRM, Vancouver, Canada 1997, p 1933). Expanding on this approach, a method that allows one to arbitrarily choose from among several combinations of temporal/spatial resolutions during postprocessing was developed for unilateral breast imaging (Song et al., Magn. Reson. Med. 46(3):503-509(2001)). This flexibility is accomplished by strategically interleaving multiple under-sampled projection reconstruction datasets, in which each set can be used to reconstruct a high temporal resolution image. Images with increasingly higher spatial resolutions can subsequently be formed by combining two or more interleaved datasets.
In 3-dimensional DCE imaging of breast lesions using the Song technique, it was demonstrated that various combinations of image matrix size (sampling points×number of views) and temporal resolution can be reconstructed. Using this technique, 64×64 images (using 48 projections) can be acquired every 12 seconds, 128×128 (96 projections) every 24 seconds, 256×256 (192 projections) every 48 seconds, or 512×512 (384 projections) every 96 seconds. However, the main drawback of this technique is also its strength—that is, the temporal/spatial resolution tradeoff. Due to SNR limitations and the desire for artifact-free images, high spatial resolution images required lower temporal resolution since a greater number of views were needed during reconstruction.
To simultaneously achieve both high spatial and high temporal resolutions in a single dynamic image series, Song et al., (J. Magn. Reson. Med. 52(4):815-824 (2004)) used a weighted radial view sharing scheme (KWIC) that preserved spatial resolution, temporal resolution and image quality.
As the value of DCE-MR of the breast is appreciated by clinicians, its usage is likely to increase, and there will be a demand for bilateral breast acquisitions. However, the problem of imaging at a high frame rate while preserving spatial resolution is compounded in the case of bilateral imaging. In a clinical bilateral exam the acquisitions are often interleaved, which doubles the scan repetition rate (TR) and reduces the temporal resolution. A coarse representation of the contrast kinetics is the best that can be achieved with this type of acquisition. In another approach often used clinically, the breasts are scanned with individual unilateral studies on consecutive days with a significant penalty in cost and patient inconvenience.
As a result, while these methods offered a significant improvement, alone they cannot achieve the temporal resolution needed to adequately sample the enhancement curve, while simultaneously acquiring high spatial resolution images of both breasts. Thus, until the present invention, there has been a need for an effective and more accurate method for the bilateral screening of both breasts, particularly for use in high-risk screening, cancer staging, and for potentially reducing the number of breast biopsies. Availability of an easy to employ, more accurate methodology for such testing will lead to vast improvement in early and accurate diagnosing with the lowering of the morbidity and mortality of breast cancer.